1. Field of the Invention
The present invention relates to a tuner circuit employed in a high-frequency circuit device in a television receiver or a video tape recorder, and particularly to a tuner circuit having a local oscillator circuit including a coaxial resonator.
2. Description of the Background Art
Referring to FIGS. 1 to 3, description will be given for a conventional double-conversion tuner. Referring to FIG. 1, the double-conversion tuner includes an input terminal 100 for inputting an RF (Radio Frequency) signal, an RF bandpass filter 101 for extracting the required frequency component for the RF signal, a broadband amplifier 102 for amplifying the RF signal, a clock signal input terminal 106 for inputting a clock signal, a data signal input terminal 107 for inputting a data signal, a PLL (Phase Locked Loop) circuit 105 for accurately obtaining a first local oscillator signal L01 described below, a first local oscillator circuit 104 connected to PLL circuit 105 for generating the first local oscillator signal L01, a first mixer 103 for mixing the RF signal output from broadband amplifier 102 with first local oscillator signal L01 output from first local oscillator circuit 104 to generate a first IF (Intermediate Frequency) signal, a bandpass filter 108 for extracting required frequency component of the first IF signal, an AFT terminal 114 to which an output voltage of an AFT (Automatic Fine Tuning) detection circuit described below is applied, a second local oscillator circuit 110 connected to AFT terminal 114 for generating a second local oscillator signal L02, a second mixer 109 for mixing the first IF signal output from bandpass filter 108 and second local oscillator signal L02 output from the second local oscillator circuit for generating a second IF signal, a bandpass filter 111 for extracting the required frequency component of the second IF signal, a first IF amplifier 112 for amplifying the second IF signal, and an output terminal 113 for outputting the second IF signal.
The RF signals input from input terminal 100 are input to first mixer 103 via RF bandpass filter 101 and broadband amplifier 102. RF signals are mixed further with first local oscillator signals L01 corresponding to a selected channel from first local oscillator circuit 104 and are converted by frequency into the fist IF signals. Unwanted signals other than those at the selected channel are eliminated from the first IF signals by bandpass filter 108 and are input to second mixer 109. The first IF signals are mixed with second local oscillator signals L02 and are converted by frequency into the second IF signals. The second IF signals are output from output terminal 113 via bandpass filter 111 and IF amplifier 112.
The second IF signal has its phase detected by an AFT detection circuit not shown which is connected to the following stage. This voltage is applied to an AFT terminal 114 so as to correct the oscillation frequency of second local oscillator signal L102 such that the second IF signal would be at a normal frequency. Accordingly, the influence of change in the oscillation frequency owing to temperature and secular change, and the influence of offset in the sent RF signals are corrected.
Referring FIG. 2, the second local oscillator circuit 110 includes a coaxial resonator 115, a transistor Tr for oscillation, bias resistors R3, R4 and R5, a junction capacitor C1, feedback capacitors C5 and C6, a coupling capacitor C7, and a ground condenser C8, a bias resistor R2, a bias resistor R1, and a power supply terminal B.
Coaxial resonator 115 includes a Z0 trimmer 201 for adjusting oscillation frequency, a central conductor 200, a variable capacitance diode D, tuning capacitors C2, C3 and C4.
Specifically, second local oscillator circuit 110 is formed of a modified Colpitts oscillation circuit in which transistor Tr for oscillation has its base in junction with central conductor 200, tuning capacitors C2, C3, C4 and variable capacitance diode D, employing a reduced .lambda./2 coaxial resonator. The cathode of variable capacitance diode D is connected to power supply terminal B via bias resistor R1. Meanwhile, the anode of variable capacitance diode D is connected to AFT terminal via bias resistor R2.
Referring to FIGS. 3A and 3B, central conductor 200 and Z0 trimmer 201 are respectively formed by punching a metal plate in U-shape, as shown in FIG. 3A. They are inserted into a printed board which is to be the main substrate of the tuner, and are fixed by soldering. As shown in FIG. 3B, adjustment of oscillation frequency for correcting offset thereof owing to variation of the components forming the oscillation circuit had been effected by tilting Z0 trimmer 201 toward or away from central conductor 201.
In a tuner circuit formed as described above, central conductor 200 and Z0 trimmer 201 respectively have different shapes and masses, and are mounted onto printed board 202 independently. Accordingly, when the entire tuner is subjected to oscillation, central conductor 200 and Z0 trimmer 201 respectively oscillate in different modes. Thus, howling occurs, causing failures in reception such as disturbance in video information and generation of noise.
In order to prevent this occurrence of howling, central conductor 200 and Z0 trimmer 201 are respectively fixed by paraffin or silicon adhesive. When this is done, however, influence of shrinkage of the adhesive due to its curing and the like may cause offset in the oscillation frequency to degrade its accuracy. To deal with this problem, adjustments were carried on by estimations, estimating the degree of change such as in shrinkage and the like in advance, but accuracy of such adjustments were limited.
Based on the foregoing, since application of adhesive is required, and since it is necessary to adjust the oscillation frequency considering the influence of the adhesive, difficulties of the process is increased, and moreover, decrease is caused in the yield, thus substantially raising the cost of the product.